The present invention relates to a light-guiding device, and is applicable particularly, but not exclusively, to a light-receiving apparatus of a laser radar system for measuring the distance between motor vehicles.
Conventionally, as light-guiding devices for measurements and the like, there are known a lens system comprising a convex lens 100, having a focal length f, and a light-receiving element 101 as illustrated in FIG. 10A and a concave mirror system comprising a concave mirror 105, having a focal length f, and a light-receiving element 106 as illustrated in FIG. 10B. The former is arranged such that light condensed through the convex lens 100 is received by the light-receiving element 101 and the later is arranged such that light condensed through the concave mirror 105 is received by the light-receiving element 106. Further, as a light-guiding device to be used for a laser radar system of a motor vehicle, there is known a device comprising a semiconductor laser element for emitting laser light through a light-transmitting/receiving system convex lens toward an object and a reflecting plate having a reflecting surface to reflect the laser light emitted therefrom or the laser light reflected from the object as disclosed in the Japanese Utility Model No. 3-14478.
In the aforementioned conventional light-guiding devices, because of having to guide light through the convex lens 100 or the concave mirror 106, a space is required between the convex lens/concave mirror and the light receiving element 101/106 which corresponds to the focal length f light transmitting/receiving system. Hence, there is a problem that the device is required to be enlarged in size in the thickness direction. This problem is also presented in the light-guiding device as disclosed in the Japanese Utility Model No. 3-14478. One possible solution of this problem is to use a convex lens 100 having a shorter focal length as compared with the aperture. However, difficulty is encountered to effectively condense light because of large affection of the aberration.